When a ship is loaded no cargo or a small cargo, the ship becomes difficult to maintain its equilibrium because of a water line is lowered. Therefore, such a ship is loaded with fresh water or seawater as ballast water to maintain its equilibrium thereby assuring safe navigation thereof in the ocean. The ballast water is taken in the ship at an unloading port before shipment and is discharged outside the ship before the ship comes into a loading port or at the time of loading a cargo.
The fresh water or the seawater as the ballast water is taken in a watertight compartment constituted inside the ship by a pump or the like and held there. On this occasion, various types of microorganisms such as plankton and bacteria and aquatic organisms such as minute shells viable in an intake area are taken in. By discharging such ballast water as described above, for example, along the coast near the loading port or in the loading port, there is caused a problem of damaging ecosystems in a surrounding ocean area. Further, since the ballast water is held for a long period of time in a closed light-shielded condition, an amount of dissolved oxygen is decreased. By discharging such ballast water as having a poor oxygen (reducing) condition, there is caused a concern as to whether a deleterious effect is given to organisms in the surrounding ocean area.
As described above, since the ballast water is held for a long period of time in a dark reducing condition, plankton or aerobic bacteria which require light or dissolved oxygen are hardly viable in the ballast water and cyst in which plankton is in a dormant state or anaerobic bacteria tend to grow. As for the cyst, an exterior wall thereof is completely different from a cell wall membrane of the plankton, has an extremely hard structure and, then, has an extremely high durability.
Under these circumstances, the present applicant has proposed a method for destroying cyst of noxious plankton by maintaining such an amount of hydrogen peroxide or a compound producing hydrogen peroxide as being effective in destroying the cyst of the noxious plankton in ship ballast water (Japanese Examined Patent Publication No. 2695071: Patent Document 1).
Further, there has been proposed a method for sterilizing ship ballast water by killing cyst of noxious algae by adding a chlorine type biocide or hydrogen peroxide to the ship ballast water (Japanese Unexamined Patent Publication No. HEI 4(1992)-322788: Patent Document 2). However, in a case in which the chlorine type biocide is added, a trihalomethane is generated in the ship ballast water, to thereby generate a concern about an environmental problem.
Further, there has been proposed a method of adding metallic powder such as iron powder as an antioxidant such that a concentration of dissolved oxygen and/or carbon dioxide is reduced to a level at which organisms viable in the ship ballast water are no more capable of being viable (Japanese kohyo No. 2000-507153: Patent Document 3). However, when the concentration of the dissolved oxygen in the ship ballast water is reduced to a level at which the organisms are incapable of being viable, by discharging the ship ballast water, there is caused a concern as to whether a deleterious effect is given to organisms in the surrounding ocean area.
On the other hand, there have been proposed various types of other methods for exterminating organisms in the ship ballast water than those of adding chemicals. For example, a method in which an electrode is disposed in a ballast tank and, then, electricity is turned on (Japanese Unexamined Patent Publication No. 2001-974: Patent Document 4), a method in which a pair of electrodes are disposed in a passage of ship ballast water and, then, an electric field is generated (Japanese Unexamined patent Publication No. 2003-334563: Patent Document 5), a method in which ship ballast water is heated for a given length of time (Japanese Unexamined Patent Publication No. HEI 8(1996)-91288: Patent Document 6), a method in which a nitrogen gas is supplied in ship ballast water tank (Japanese Unexamined Patent Publication No. 2002-234487: Patent Document 7) and a method in which steam is injected in ship ballast water tank, the steam and ozone are injected in the ship ballast water tank, or an ultraviolet ray is irradiated while injecting the steam (Japanese Unexamined Patent Publication No. 2004-160437: Patent Document 8) are known. However, each of these methods requires an apparatus of large scale and, accordingly, it is practically difficult to apply any of these methods to an existing ship.
Patent Document 1: Japanese Examined Patent Publication No. 2695071
Patent Document 2: Japanese Unexamined Patent Publication No. HEI 4(1992)-322788
Patent Document 3: Japanese kohyo No. 2000-507153
Patent Document 4: Japanese Unexamined Patent Publication No. 2001-974
Patent Document 5: Japanese Unexamined Patent Publication No. 2003-334563
Patent Document 6: Japanese Unexamined Patent Publication No. HEI 8(1996)-91288
Patent Document 7: Japanese Unexamined Patent Publication No. 2002-234487
Patent Document 8: Japanese Unexamined Patent Publication No. 2004-160437